Drain and rinse beans in colander and transfer to a large bowl.
Combine olive oil, lemon juice and salt and pepper and pour over the
beans. Add the onions and parsley and mix together. Allow sitting for
an hour or so to let the flavors blend. Transfer the beans to a
serving platter. Drain tuna, break it into chunks and arrange it on
the beans. Garnish with sliced black and green olives and serve.